This invention relates generally to the use of an air-bearing to sense longitudinal tension exerted upon a thin, flexible, elongated plastic bubble which is continuously moving relative to the sensor. More particularly, this invention relates to a tension sensor suitable as a process control device in the manufacture of thin plastic film. Typically in the manufacture of such film, appropriate feed material at an elevated temperature is continuously extruded through an annular die, partially cooled and subsequently taken off by rollers downstream of the die. By pinching the partially cooled extruded film with the downstream rollers and pressurizing the space defined by the film between the die and the pinch rollers, a large bubble may be formed.
By selectively varying the tension exerted by the rollers on the hot extruded film bubble, the film may be partially oriented such that its mechanical properties are selectively varied. Further, it is important to have the capability to sense tension fluctuations about a norm selected to produce a film having relatively uniform orientation and predetermined selected properties. The present invention permits such fluctuations to be measured directly from the film material by utilizing a sensor having an air-bearing element.
An air-bearing having a porous element of sufficient permeability such that an air flow passing through the element will support a member moving over the effluent surface of the porous element is selected to carry out this invention. The flow of air uniformly emanating from the porous surface provides an air cushion that acts as a practically frictionless load-bearing surface. A more detailed discussion of this type air bearing is found in "Introduction to Engineering Controlled Porosity Products" and "Air Film Rolls" published by the Mott Metallurgical Corporation, Farmington Industrial Park, Farmington, Connecticut, Catalogs 1000 and 3000, respectively.